Doping with Chemically Hard Elements to Improve Photocatalytic Properties of ZnO Nanostructures

Here we report improved photocatalytic activity of ZnO nanostructures through judicious choice of the dopant, particularly by choosing the dopant with high chemical hardness. We have doped ZnO nanostructures with divalent (Ni2+ and Sr2+) and trivalent (Cr3+ and Al3+) using hydrothermal conditions. The photocatalytic activity of undoped and doped ZnO nanostructures was tested towards the degradation of methyl orange (MO), methylene blue (MB), and congo red (CR). The Al-ZnO exhibited a 91.25% removal rate compared to 40.88% for pure ZnO in MO degradation under ultraviolet (UV) radiation. The trend of photocatalytic activity of different photocatalysts (Al-ZnO > Sr-ZnO > Cr-ZnO > Ni-ZnO > ZnO) is similar to the trend of Pearson hardness values of Al3+ > Sr2+ > Cr3+ > Ni2+ > Zn2+. These reactions also reveal that the dopant type affects the morphology of the ZnO nanostructures in given reaction conditions. We propose the polarity-induced mechanism to understand the changes in the morphology due to different dopants.